Fire Resistant Steels
Fire Resistant Steels
Fire is a chemical phenomenon which occurs as a result of thermal processes. When a steel section is exposed to a fire then the level of temperature increase on the face of steel section depends on thermal inertia, exposure of surface area, and the protective coating. As the rate and quantity of heat flow from the fire environment to steel section increase, the temperature, and thus the risk of failure for the steel section also increases.
There are different means to assure fire safety of steel constructions. Traditional fire protection systems, such as plasterboard, spray or intumescent coating, are proven and tested. However, these systems are linked with additional working processes and hence additional costs. In contrast to this, fire resistant steels come with inherent fire protection achieved by modified alloy. Hence, the user can save considerable cost. Further, the steel structure remains visible and hence architecturally appealing. Fire resistant steel allows for fast erection of buildings with the use of standardized connections. Besides, usable space is increased as fire protection can be omitted.
In the aftermath of the World Trade Centre disaster, further aspects as robustness of fire protection and its integrity has been spotlighted. While the traditional fire protection can be removed and is prone to damage, this is not the case for fire resistant steel. In spite of these advantages, the rare use of fire resistant steel bewilders at first glance. On closer examination of the reasons, it becomes clear. Both the normative regulations and material properties for fire resistant steel are missing.
Structural fire safety is one of the primary considerations in the design of high rise buildings where steel is frequently the material of choice for structural members. At present, structural fire safety (fire resistance) of steel members is normally achieved through prescriptive approaches which are based on either standard fire resistance tests or empirical calculation methods. These prescriptive based approaches have major drawbacks and do not provide a rational or realistic fire resistance assessment. The recent move towards performance based fire design has increased the focus on the use of computer simulations for evaluating fire resistance of structural members. Knowledge of high temperature properties of steel is critical for evaluating fire resistance using numerical models.
Since the steel has a very high thermal conductivity, exposed surface of the steel section easily transmits the conveyed heat from the fire source to the other members of the whole structure in a short period of time. Heat is transmitted in between the steel sections from high temperature sections to low temperature sections by way of conduction, radiation, or convection modes. Steel sectional properties and its yield strength considerably reduce as it absorbs heat upon exposure to a high temperature level.
A steel structural member can easily collapse during a fire if the temperature is allowed to reach a critical value. The fire resistance of the steel member is related to some important factors including the section size, the perimeter of the section exposed to fire, place of the member in the protective structural assembly, and the steel material thickness. Depending upon structural assembly and the fire occurrence, even the exposed steel can resist fire up to 30 minutes.
The fire resistance of structural steel normally used in building constructions is usually not high enough to ensure that all external and internal loads applied to such structures under fire conditions are safely carried if the members made of such a steel is not adequately protected from direct fire exposure. Hence, both the active as well as the passive fire protection measures, which are relatively expensive and not always sufficiently effective, are to be used in engineering practice in order not to exceed the acceptable risk level of the building damage in case of a fire. However, there is the possibility of an alternative approach to provide the required fire resistance of steel structures. In this approach, the result is achieved by replacing the traditional types of structural steel by the special fire resistant steel grades with a carefully selected chemical composition. It is necessary that the steel of this type is to be characterized by the yield limit which under fire conditions becomes considerably higher than the analogous yield limit relating to the conventional structural steel.
Fire resistant steels have been developed for construction applications where increased high temperature strength provides improved protection to a building structure during a fire. Improved fire protection, in turn, helps to prevent building collapse caused by reduced load carrying capability of steel structures at high temperature, or provides the building occupants greater time to escape the building in the event of such a collapse.
However, structural steel needs to be protected against fire using the proper insulating materials and methods to control the situation and resist for longer periods. Normally the structure’s fire safety is measured by the time of resistance regarding the supplied evacuation time and the level of failure. It also is important to assure that the fire resistance time is sufficient as such that the structure is able to carry the building loads during the duration of the fire.
There are the basic structural mechanisms by which it is possible to achieve the sufficient fire resistance of structural steel on its direct exposure to a fire. It can be seen that the obtained benefits depend on an appropriate adjustment of the chemical composition of the considered steel, especially by rational selection of alloying elements used in practice, although the effect achieved as a result of such activity is normally complex and frequently multi-faceted.
Presently fire resistant steels are not the merely experimental curiosities. They are widely used for many types of the building structures, such as for drive-in-multi-level car parking lots, for roof frames designed for atria, railway stations, sports facilities like gymnastic halls, and many other structures. An interesting example of a multi storey building with the skeletal load bearing structure made entirely of the fire resistant steel is Tobihata building completed in Japanese Kitakyushu as early as at the beginning of the nineties of the last century. Another example worth noting in this area can be the fire resistant steel structure of the Hokke Club Ikenohata hotel building (at present known as the Sofitel hotel), located in Tokyo. The use of fire resistant steels successfully reduces the quantity of fire protection of the building structures and the structural members can be even unprotected in cases where the steel temperature does not exceed 600 deg C.
For improvement of the fire resistance properties of steel structures, a good quantity of work has been done and a number of fire resistant steels with different strength levels have been developed. These steels are much superior to the conventional steels with regards to the high temperature yield strength. The yield strength of the fire resistant steels at 600 deg C is normally set at the minimum of two thirds of the specified room temperature yield strength while maintaining low yield ratio, good weldability, and other properties.
In the earlier studies, it has been suggested that an attractive approach to production of structural steels involves the use of niobium and molybdenum additions. This is to increase the high temperature strength by stabilizing the microstructure at high temperature through solute and precipitation effects and by dispersion strengthening through fine alloy carbide precipitates containing niobium and molybdenum. Other elements such as tungsten, vanadium, chromium, and titanium etc. are also relevant to fire resistant steels.
A recent investigation following the niobium – molybdenum approach has suggested a contribution from the base microstructure of the ferrite, associated with alloying effects on the austenite transformation behaviour during cooling. In particular, an acicular ferrite or bainite microstructure is suggested to improve strength retention at temperatures up to around 700 deg C, primarily because of finer and more closely spaced alloy carbides resulting from an increased number of nucleation sites associated with the high dislocation density of the bainite.
Steel is inherently a non-combustible material. It loses strength when heated sufficiently. Steel structural properties and its yield strength considerably decrease when it is heated to temperatures seen in a fire scenario. The critical temperature of a steel structural member is the temperature at which it cannot safely support its load. Building codes and structural engineering standard practice defines different critical temperatures depending on the structural element type, configuration, orientation, and loading characteristics. The critical temperature is frequently considered temperature at which its yield stress has been reduced to around 2/3 of the room temperature yield stress.
Fire resistant steel for building structural use, which Nippon Steel Corporation developed in 1988 for the first time in the world, is characterized by markedly enhanced fire resistance properties such as high temperature strength. Presently it is widely used for different structures.
The earliest study associated with the designing of the fire resistant structural steels was that carried out by the French steel company Creusot-Loire, in the 1970s. This was a very unsuccessful venture, because of the lack of understanding of the effect of thermo-mechanical treatment of the steel on its high temperature properties. The next experiments in this area were conducted by the Australian steel company Broken Hill Proprietary (BHP). In these steels, the high temperature strength was correlated with microstructure of acicular type, combined with a secondary precipitation hardening component (vanadium and / or niobium). Since the early studies on high temperature strengthening mechanisms were concentrated mainly on pure ferritic structures, so the investigation on the effect of acicular microstructure was very innovative at that time.
The end of the twentieth century with respect to the study how to design the effective fire resistant structural steels was connected to the strong dominance of some Japanese organizations, such as Nippon Steel and to a lesser extent Kawasaki Steel and Sumitomo Metals. In these experiments, the Nb(C,N) (niobium carbo-nitrides) precipitation as well as the solid solution of molybdenum were analyzed as the basic strengthening mechanism. The Nippon fire resistant steels were further improved at the Queen’s University of Belfast in Northern Ireland. It was confirmed that in these steels both precipitation and solutes with a large atomic misfit provide the necessary strength retention. This is the effect of the ultra-fine precipitates, which are slow to coarsen.
Though the fire resistant steel developments have been led by activities in Japan, studies are also available from Europe, China, Korea, and the USA. The steels are intended to resist accelerated creep, or thermally activated deformation. The term ‘accelerated’ creep is used here to distinguish the fire resistant steel application from other creep sensitive applications involving exposure to high temperatures and stresses for much longer durations (months or years) than apply to building fires, where locally high temperatures are more commonly encountered over time periods lasting up to a few hours.
The goal in fire resistant steel development consists of employing strengthening mechanisms which maintain superior effectiveness at high temperature, hence providing resistance to softening. It is to be noted that long duration creep behaviour involves a different deformation mechanism, and some of the understanding of creep strengthening mechanisms is less applicable to the ‘accelerated’ creep mechanism applicable to fire resistant steels. As a result, much of fire resistant steel development has been somewhat empirical so far, directed toward meeting specific performance qualities, and there remains an opportunity to develop further understanding of the basic mechanisms to provide the tools for more efficient steels and process design optimization in the future.
One of the most important characteristics of fire resistant steel is that its yield strength at 600 deg C is 2/3 or more of the specified minimum yield strength at room temperature. By virtue of its good high temperature properties, fire resistant steel has made it possible to reduce or eliminate fire resistant coating for steel frames, which was compulsory under the ‘building standard regulation’ in the past, and as a result, considerably reduced the building construction costs and period, and facilitated attractive building design.
Defining more precisely, according to the requirements developed by the Japan Institute of Metals, the yield point of the fire resistant steel relating to the steel temperature equal to 600 deg C is to be ensured to be in the range of two thirds of that specified at room temperature. This restriction is much stronger than the analogous one, specified in the USA, as per which only at least one half of a room temperature yield strength is needed as the remaining in such cases. In addition to this basic requirement, the weldability of such a fire resistant steel during the whole time of a fire exposure has to remain similar to that relating to the conventional steel not exposed to a fire.
It is common knowledge that there are at least several approaches to effectively obtain a structural steel having such properties. They normally deal with the identification of the recommended method of the steel production, such that its preferred microstructure can be obtained. In general, it is reasonable to claim that a mixed steel structure, of bainite and ferrite, is effective in achieving a low yield limit reduction, especially with a high strength steel. However, a special attention is needed to be paid to the relationship between the applied chemical composition of the considered steel and the material characteristics obtained as a result of its use. In particular, the effectiveness as well as the desirability of the use of various type of alloying elements and other admixtures are to be considered in detail in terms of their suitability to achieve the intentional properties of steel.
Fire resistant steel has higher yield strength at higher temperatures when compared to carbon steels. Japanese Industrial Standard (JIS) specifies that the 0.2 % proof stress of fire resistant steels at 600 deg C must retain at least 2/3 of the value at ambient temperature. In other aspects, at ambient temperature, fire resistant steel has the performance and weldability similar to carbon steel. Basically, fire steel has similar chemical composition to carbon steel of same strength grade, but has additions of niobium, molybdenum and other alloying elements to improve the yield point at high temperatures. Fig 1 shows comparison of the influence of temperature on the properties of fire resistant steels with carbon steel. In the Fig 1, KSFR and NSFR represent the fire resistant fire resistant steels developed by Kawasaki steel and Nippon steel respectively.
Fig 1 Comparison of influence of temperature on the properties of fire resistant steels with carbon steel
Steel has good strength properties at ambient temperature, however, like other materials, steel loses its strength and stiffness with temperature. The temperature dependent properties which are important for modeling the fire response of steel structures include thermal, mechanical, and deformation properties. These properties vary with temperature and are also dependent on a number of parameters. Much of the present knowledge on the high temperature properties of steel is based on limited material property tests. Also, the presently available information for high temperature material properties is only for the heating phase of fires. This is since most, if not all, material tests are conducted under either transient or steady state tests with increasing temperature. There is a lack of data on material properties in the cooling phase, and this is critical for modeling the response of steel structures under realistic fire.
Earlier, for example, fire protection was assured in a 1969 requirement that structural steels not exceed a temperature of 350 deg C. It has been considered that the yield strength of conventional steels at 350 deg C is around 2/3 of the specified values at room temperature, and so the fire-resistant steels developed more recently and produced the past several years ensure a minimum yield strength at 600 deg C which is 2/3 of the room temperature yield strength, i.e. having a minimum yield strength ratio of 2/3. Fig 2 shows schematic comparison showing improved high temperature strength of the fire resistant steel.
Fig 2 Schematic comparison showing improved high temperature strength of the fire resistant steel
The resistance of buildings and other facilities to fire depends on the extent to which their steel structures soften when heated to the temperatures created by the fire. A steel is normally considered fire resistant if its strength when heated to such temperatures for short periods of time remains equal to 0.6 to 0.7 of its strength at room temperature. The alloying system Cr-Mo-V-Nb can be used for steels that are designed to be fire resistant up to 700 deg C. The greatest resistance to fire, up to 800 deg C, is obtained in steels which contain boron.
Accurate prediction of the material properties of fire resistant steel is crucial for determining the load bearing capacity of fire exposed structures. Moreover, the scope of application for the fire resistant steel is not clear, e.g. if it is suitable for members in bending as well as members endangered by global instability.
Structural mechanisms affecting the steel resistance under fire temperature
The strength of a steel is normally determined by the strength of its metallic matrix. It is a well-known fact that such strength decreases with the material temperature growth. This happens due to the increasing dislocation mobility, which becomes activated by the thermal energy supplied to the matrix. The stress necessary to move a free dislocation in a metallic matrix is called the Peierls-Navarro stress. It is obvious that its value is strongly temperature-dependent since the thermal agitation is helpful in moving dislocations around the short range obstacles. If it is underlined that at room temperature, these dislocations are normally constrained to move only in certain glide planes then, however, at high temperature diffusion of the point defects can change the outline of a dislocation, enabling an edge dislocation to move around an obstacle. Such a process is normally called ‘the dislocation climb’.
The second process, named ‘the dislocation cross-slip’, occurs when a screw dislocation moves to a parallel slip plane using an intersecting slip plane of the same system. Recently, it has been reported that the dislocation cross-slip can occur at lower temperature value in relation to that which is needed to initiate the dislocation climb.
The precipitation can be another process affecting the stability of a metallic matrix. In alloy systems, it occurs heterogeneously since the precipitate coarsening is accelerated on dislocations and grain boundaries because of the faster diffusion of solute elements along these channels. However, it can be initiated. In such conditions the interface energy of the precipitate / matrix interface is lowered and a more stable configuration is achieved in a considered metallic matrix. Dispersion of precipitates can result in the strengthening of metallic matrix provided that their size is not too big. However, this effect strongly depends on such a size and on the inter-particle spacing in a matrix.
Hence, if a critical size of the precipitates is surpassed and, furthermore, if the inter particle spacing in metallic matrix is sufficiently increased as a consequence of the impact of high temperature, the contribution of this process to the strengthening of the considered structural steel starts to decrease and finally becomes disadvantageous.
Weakening of a steel strength can also be caused by creation of the vacancies in metallic matrix when some atoms are missing from one of the lattice sites. These vacancies are the unfavourable point defects which are frequently interpreted as the so called Schottky defects. Such creation is considerably intensified at fire temperature since the thermal agitation moves all atoms from their atomic sites. However, it is obvious that few vacancies can occur even at much lower temperature. As it has been shown in the experiments, around one site in 104 becomes vacant near the steel melting point. Further, the creation of the vacancies on dislocations in metallic matrix decreases the effect of precipitates at high temperature by enhancing the process of dislocation climb.
The detrimental effects on the strength of structural steel have also the impurity inclusions. Such effect is noticeable even at room temperature when these particles detach from the matrix soon after the plastic yield begins. However, at high temperature it is much more damaging because of a different coefficient of thermal expansion characterizing the particles of the impurity in relation to that assigned to iron. This difference is the source of the thermal stresses induced in a metallic matrix as a result of the impact of fire temperature, leading to the premature de-cohesion. It is a well-known fact that at room temperature de-cohesion occurs first at MnS (manganese sulphide) inclusions, then at smaller oxides and finally at small carbides. Also, it has been stated that calcium aluminates and alumina are the most detrimental in this field because of their substantially smaller coefficient of thermal expansion than that relating to the parent steel.
The next process affecting the strength of the considered steel at high temperature is the grain boundary sliding. As recently has been reported, such sliding becomes substantial when the steel temperature reach the value of 630 deg C. This temperature is known as the equi-cohesive value since at this level, the grain boundary strength and the matrix strength are equal. If the temperature exceeds this level, the grain boundary is weaker than the grain interior. The grain boundary sliding occurs by a shear process along the direction of the boundary. It is normally associated with creep deformation. However, its contribution to the overall matrix deformation during the relatively short lasting fire is estimated as almost non-existent. In fact, creep defined as the progressive deformation of a material matrix at constant stress, which is frequently less than the material yield stress and is normally considered to be irrelevant in fire because of the relatively short duration of such a fire. However, some processes being thermally activated, included those listed above, can become much more intensified when they occur in the interaction with creep. In particular, this refers to dislocation glide, diffusion creep, and grain boundary sliding.
When a metal is alloyed with another metal, either substitutional or interstitial solid solutions are normally formed (Fig 3). Substitutional solid solutions are those in which the solute and solvent atoms are nearly the same size, and the solute atoms simply substitute for solvent atoms on the crystalline lattice. On the other hand, interstitial solid solutions are those in which the solute atoms are much smaller and they fit within the spaces between the existing solvent atoms on the crystalline structure. However, the only solute atoms small enough to fit into the interstices of metal crystals are hydrogen, nitrogen, carbon, and boron. The other small diameter atoms, such as oxygen, tend to form compounds with metals rather than dissolve in them. When both small and large solute atoms are present, the solid solution can be both interstitial and substitutional. The insertion of substitutional and / or interstitial alloying elements strains the crystalline lattice of the host solvent structure. This increase in distortion or strain, creates barriers to dislocation movement. The distortion energy causes some hardening and strengthening of the alloy and is called solid solution hardening. It can be highlighted that interstitial atoms normally strengthen a metal matrix more than substitutional atoms do, since the interstitials cause more distortion.
Fig 3 Substitutional and interstitial solid solution of alloying element
Both interstitial and substitutional solute atoms can associate with dislocations in order to lower their strain energy. This means the strong interaction between such atoms and the strain fields generated around these dislocations. However, the association of elements to dislocations becomes unstable at high temperature. For example, at a certain critical temperature, the carbon atoms can diffuse away from the dislocations, causing the disappearance of the yield point above this temperature. It is necessary that the steel yield point can return to the value previously specified at lower temperature due to the segregation of carbon and nitrogen atoms to the dislocation cores. This process is known as strain ageing. As it has been reported in a study, the kinetics of strain ageing are highly dependent upon the interstitial content. The return of the steel yield point is accelerated if the interstitial content increases, ranging from months at low interstitial contents down to a few hours at high interstitial contents.
Influence of alloying elements on the resulting high temperature resistance of structural steel
To effectively choose the type and the content of alloying elements which, when added to the chemical composition of conventional structural steel, result in an increased resistance to its exposure to a fire temperature, it is necessary to recognize the mechanisms of actions of these elements on the resulting behaviour of the heated steel. In further analysis each of these mechanisms, relating to the particular elements, is described in detail below.
Carbon and nitrogen – It is widely recognized that carbon and nitrogen are two of the most influential elements determining the high temperature strength of structural steel. Both have an essential effect on the steel yield point as well as on the steel ultimate stress when they are in solid solution, even if these values are related to the room temperature. Such behaviour results from the occupation of the tetragonal interstitial site in alpha iron body-centered cubic lattice structure. It is reported that the resultant large strains of metallic matrix give a strengthening increment of 5,500 MPa computed per 1 % by mass. However, the size of such an increment is limited by their low solubility in ferrite. This solubility is estimated to be equal to 0.02 % for carbon when it is at the temperature 723 deg C and to 0.1 % for nitrogen when it is at the temperature 590 deg C. As per a study, the increase in the strength has been observed also at the temperature up to 450 deg C for 0.12 % carbon content but the mechanism of this strengthening has not been thoroughly studied. Also, it has been reported that the interstitials have little effect on creep resistance at the temperature above 450 deg C. The second mechanism identified with the increase of the steel yield stress is that relating to the segregation of carbon and nitrogen atoms to grain boundaries, where they pin grain boundary dislocations.
As per a study, the conventional way to improve moderately the strength of the steel at high temperature (i.e. up to 350 deg C) is by increasing the carbon content. It is to be noted, however, that greater carbon content in hot rolled steel normally worsens both its notch toughness and its weldability. For this reason, carbon content is normally limited in structural steels to a maximum of 0.2 %. Hence, for fire resistant steels which can be safely used under direct fire exposure, a practical carbon range of 0.05 % to 0.2 % is designed.
Manganese – As it is normally known, deoxidation is the primary function of manganese in structural steels. Another function is to prevent hot shortness by removing sulphur through the formation of MnS inclusions. Also, it is used to reduce the formation of grain boundary cementite. It also can produce temper embrittlement. Manganese increases the strength of the steel by solid solution hardening in hot rolled steels. The strain hardening interaction between manganese, nitrogen and carbon is well known at temperature range of 250 deg C to 500 deg C, which produces a moderate strengthening effect. Also, manganese tends to lower the eutectoid carbon content in the steel and promotes segregation of carbon and nitrogen atoms.
Silicon – Silicon is a solid solution strengthener and also a deoxidizer. In structural steels, it is a mild ferrite former and solution hardener of ferrite. Appropriate addition of silicon to the steel chemical composition results in a higher high temperature strength of such a steel by retarding the coarsening of cementite. As silicon increases the activity of carbon and nitrogen atoms, the levels of grain boundary segregation of these elements are reduced. Also, silicon retards softening of the steel at high temperature and hence increases its hardenability. If the silicon content in the steel exceeds the level 0.3 %, then it can adversely affect the weldability of this steel.
Molybdenum – Molybdenum is considered as the element essential for high temperature applications of all types of structural steel but it is especially advantageous in relation to the low alloy steels. In fact, it is used in most of the fire resistant steels presently being produced all over the world. It considerably increases both the tensile and the creep strengths of such a steel at fire temperature. Also, molybdenum increases the solubility of niobium in austenite, thereby the precipitation effects of carbides, i.e. Nb(C, N), in ferrite are also increased. However, it seems that the basic mechanism associated with the molybdenum activity and strengthening the steel under fire temperature is that relating to the annihilation of dislocations creating in the metallic matrix. Such dislocation locking occurs in association with carbon and nitrogen atoms.
Also, the formation of a layer of molybdenum atoms around NbC precipitates reduces the rate of coarsening by slowing the diffusion of niobium atoms to the precipitate. Molybdenum tends to obstruct self-diffusion of iron hence increasing the recrystallization temperature of a steel. Another mechanism relating to molybdenum activity and increasing the high temperature strength of a steel under fire exposure deals with the formation of molybdenum carbo-nitride. The molybdenum atoms can also form clusters, which have a similar effect to precipitates.
Vanadium – It is a well-known fact that vanadium is one of the main alloying elements identified in the chemical composition of conventional grade structural steel. It is also frequently added to some of the higher-carbon constructional steels. Such an addition is very favourable if the steel is to be used as the fire resistant steel since it increases the steel resistance to temperature, provides the tempering of such a steel as well as retards the growth of carbides at service temperature. This strengthening effect is achieved due to the precipitation of vanadium carbo-nitride.
Niobium – Niobium is another element, used in combination with molybdenum, which is essential to increase the high temperature strength for low alloy steels. This effect is a result of the fact that niobium is a very strong carbide former. Also, such carbides are more stable than those formed by molybdenum. Hence, the main role of niobium is to be a grain refiner in austenite. Through the interaction between niobium and molybdenum the NbC precipitation can be delayed so as to overlap after the transformation from austenite to ferrite during cooling. Niobium also has a considerable solid solution strengthening effect, especially at high temperature. For example, it is reported that the strength of a ferritic stainless steel at the temperature 950 deg C is directly proportional to the quantity of niobium in solution, up to the level of around 0.8 %, where a peak of such strength is reached.
Chromium – The effect of chromium to the steel strength achieved at the fire temperature is complex, especially when it is alloyed together with molybdenum or vanadium. Chromium is also a carbide former. Also, it has a faster diffusion rate in ferrite than most metallic elements. Owing to this, the precipitation of chromium carbides can occur already when the temperature is not very high. For example, for Cr7C3 it is possible if only the temperature is as low as 500 deg C. The rapid coarsening is unfavourable in this case. However, molybdenum is normally being added to reduce the rate of such coarsening.
The solubility of chromium in ferrite is up to 100 %. Also, chromium can dissolve in Mo2C and make it less stable, by decreasing the crystalline lattice parameter, moving the secondary hardening peak to lower temperature values. To sum up, the principal functions of chromium are those relating to increase of a steel hardenability as well as to give some contribution both to the high temperature strength and to the oxidation resistance of such a steel.
Cobalt – The basic function of cobalt is to resist softening of a steel at high temperature when dissolved in either ferrite or austenite. Due to this, it has a little effect on hardenability of such a steel. It is also a ferrite stabilizer. Cobalt has 80 % solubility in ferrite; however, its carbide forming tendency is only slightly higher than that of pure iron.
Experimental evaluation of the benefits arising from the use of alloying elements on the achieved fire resistance of structural steel
As per one of the studies, the strengthening mechanisms which are normally identified as those that determine the properties of the considered fire resistant steel when exposed to a fire, These strengthening mechanisms are (i) pure solid solution strengthening, (ii) interaction solid solution strengthening, (iii) strain ageing, (iv) strain induced ordering of interstitial solute atoms, (v) precipitation, and (vi) ferritic grain size. Taking into account the results of several experimental studies, it appears that the interaction solid solution strengthening, between substitutional and interstitial elements, as well as the precipitation can be considered as two major high temperature strengthening mechanisms since the magnitude of these two mechanisms is considerably higher than the other contributions among the six mechanisms given above.
In general, it is reported that the mechanism relating to the pure solid solution strengthening by molybdenum or chromium is almost insensitive to temperature, provided that in the range of 20 deg C to 750 deg C. Particularly, an addition of 0.0 1% carbon or nitrogen to pure iron produce no significant strengthening in this temperature range. However, in case when the temperature exceeds the level of 450 deg C, the addition of only 0.01 % carbon or nitrogen, to binary Fe-1 %Mo and Fe-1 %Cr alloys, produce considerable strengthening, especially in the molybdenum containing alloys.
In view of the fact that these ternary alloys are effectively pure solid solutions, i.e. no precipitation occurs during testing at temperature up to at least 600 deg C, this strengthening effect is normally attributed to an interaction solid solution strengthening mechanism. Also, the effect of strain age hardening is observed in iron-carbon alloys with a little nitrogen present, at temperature higher than 200 deg C, being a result of the addition of manganese, chromium, molybdenum, tungsten, or copper. On the other hand, this effect is not achieved if vanadium or titanium is added to the chemical composition of a considered structural steel.
It is found experimentally in a study that it is possible to achieve two thirds of room temperature yield stress at 600 deg C with 0.2 % to 0.25 % molybdenum and 0.3 % to 0.55 % chromium in low carbon hot rolled structural steel. Also, enrichment the chemical composition of Cr-Mo steel by admixtures of niobium or vanadium, singly or in combination, results in its higher yield stress guaranteed at fire temperature. As per another study, the addition of 0.1 % vanadium to the 0.6 % molybdenum weathering steels has only a limited effect on the transformation of a steel whereas the addition of 0.2 % vanadium promotes the formation of polygonal ferrite. The precipitates of vanadium become finer and denser as the vanadium content increases.
As far as the precipitation of carbo-nitrides is concerned as one of the major strengthening mechanism at fire temperature its favourable effect depends on the several factors such as the composition, morphology, coherence, and stability. However the first of these factors, i.e. the composition, seems to be the most important among them since all the other factors are largely dependent on it. Also, the effect of the precipitation can be difficult to precisely estimate, especially in relation to the early studies, mainly because of the limitations at that time in the development and availability of high resolution electron microscopy (HREM) techniques.
Influence of alloying elements on the weldability of fire resistant structural steels
The carbon equivalent value (CEV) is accepted as an objective measure allowing for prediction the weldability of the newly designed fire resistant structural steels. In the application of this type the value of such a CEV coefficient can be evaluated for the assumed chemical composition of the considered steel (in percent) using the Ito-Besseyo equation CEV = C + Si/30 + Mn/20 + (Mo + Nb)/15.
If the calculated value of this factor is less than 0.4 than the predicted weldability of the examined fire resistant is good. Also, values of the CEV coefficient being under 0.35 indicate not only a good weldability but also no need for the steel preheating. As it has been confirmed experimentally in a study, all fire resistant steels presently produced all over the world are characterized by the excellent weldability properties. A very promising solution seems to be in this field is the approach based on the development of the fire resistant structural steels with improved high temperature strength which can delay the time at which the loads applied to the structure when exposed to a fire exceed the load capacity of structural members.
Alloying philosophies to develop fire resistant steels normally attempt to stabilize the initial starting microstructure and maintain effectiveness at high temperature of the strengthening mechanisms employed to meet the low temperature structural requirements, by minimizing recovery, particle coarsening, grain growth, etc. An alternate approach can be to follow a ‘smart materials’ design philosophy whereby alloying and processing are controlled to condition the initial microstructure so that additional strengthening mechanisms are activated during a fire, and some results related to this concept are mentioned below.
In general, fire resistant steels are modified versions of high strength constructional steels, normally employing micro-alloying technology, along with molybdenum additions which contribute further to the high temperature properties. A variety of alloy steels has been reported, with an emphasis on molybdenum and containing and niobium containing low carbon, low alloy steels.
In a study, the molybdenum + niobium alloy steel was used. This alloy steel was designed based on earlier development work which has led to commercial fire resistant steels with a similar chemical composition. In this earlier study, it was shown that niobium and molybdenum additions to a base carbon – manganese steel increased the high temperature strength, and that a combined molybdenum + niobium addition provided further improved properties. Other alloying approaches can also be considered, but the molybdenum + niobium approach appears to have received the most attention so far.
The benefits of combined molybdenum + niobium additions are reported to involve precipitation strengthening by both elements, and increased precipitate coarsening resistance, improving strength at high temperature. The mechanism by which molybdenum retards coarsening has been suggested to be associated with segregation to interfaces between Nb(C,N) precipitates and the surrounding matrix, reducing the precipitate / matrix interfacial energy. This interfacial energy provides the fundamental driving force for precipitate coarsening, although recent 3-D atom probe tomography work conducted on niobium-molybdenum containing steels processed at higher temperatures in the carburizing regime has not identified molybdenum segregation to precipitate / matrix interfaces of coarsening resistant niobium carbides, and hence has raised some doubts to the mechanism.
Regardless of the applicable mechanism, however, the suppression of precipitate coarsening contributes to precipitate refinement and hence to strength retention at higher temperature, and this process is considered to represent an important ‘synergy’ between elements such as molybdenum and niobium in the fire resistant steels.
Alloying also contributes to microstructural refinement, through its hardenability effects, by reducing the temperature at which the austenite phase decomposes during cooling and by promoting bainitic microstructures. This factor can be relevant in the microstructural design of fire resistant steels. While the alloy composition in the steel used in the study is similar to that of the fire resistant steels, the steel was hot rolled in production and it is to be recognized that the thermo-mechanical processing characteristics, and hence the microstructural details, are not to be identical to commercial variants.
While the high temperature tension test is technically an ‘isothermal’ test, the heating rate to the test temperature can influence the tension test results, and of course the holding time at temperature prior to testing is expected also to have an influence. A heating rate effect is notably found for a test temperature of great relevance to fire resistant steel specifications, 600 deg C. The sample is not under load during heating, so this response is not a result of creep deformation during heating, but rather shows an ‘annealing response’ associated with softening of the microstructure due to longer exposures at high temperature during heating. Softening of the microstructure can involve such mechanisms as precipitate coarsening, dislocation rearrangement (recovery) and grain growth.
Along with high temperature tension tests, another methodology has been developed to more closely simulate material response during exposure to a fire. In this test, a constant tensile load has been applied to the sample while heating at a technically constant rate. As the temperature rises, thermally activated deformation mechanisms become operative, and the sample plastically deforms when a sufficient temperature is reached, and eventually fails by ‘runaway’ strain at higher temperatures. This test is referred to here as the ‘constant load’ test, and has also been called a ‘temperature ramp’ test. While there is not a complete correspondence between the comparative results of the two tests, this test again shows the superior high temperature performance of the molybdenum + niobium steel.
Precipitate analysis has been conducted in a study after constant load testing in the niobium and the molybdenum + niobium alloys, using transmission electron microscopy (TEM) of carbon extraction replicas. The heating rate in this case has been 300 deg C per hour, a relatively slow rate where there is higher time for microstructure changes to occur during heating. It appears that the carbide precipitates in the molybdenum + niobium alloy steel are finer and occur with a much higher particle number density than in the molybdenum free alloy steel. This observation is consistent with an earlier study suggesting that molybdenum contributes to refinement of strengthening precipitates in micro-alloyed fire resistant steels.
It is to be recognized that the improvements associated with the fire resistant steels are modest, far less than the increases in allowable temperatures of several hundreds of degrees which can be expected with (much more costly) heat resisting super alloys. However, the increased performance of fire resistant steels is sufficient to justify application in some structure designs, and is expected to contribute to fire safety wherever these steels are employed. The elastic modulus is also temperature dependent, and is normally expected to be less sensitive to microstructure, chemical composition and processing than the strength, and hence there is also a (modest) limit to the benefits achievable by increasing the softening resistance of the fire resistant steels, before elastic deflection related issues become limiting.
The copper containing steel is included to determine whether precipitation during the heating event associated with a building fire can offer a strengthening mechanism to a fire resistant steel, associated with the fire itself. Such a mechanism can be considered to offer a metallurgical design concept involving a form of ‘active’ fire safety, where a potential strengthening mechanism is built into the steel itself, and only activated as a consequence of a fire. The copper containing steel is conditioned in three different ways prior to testing, which include (i) normalizing (involving cooling from high temperature, hence suppressing copper precipitation and allowing the potential for strengthening precipitates to form during heating), (Cu N), (ii) peak aging (to provide the maximum strength at the start of testing), (Cu P), and (iii) overaging (to reduce the strength at room temperature and preclude strengthening precipitates from forming during heating), (Cu O).
Only the normalized condition is expected to offer the desired precipitation mechanism described above. The test results confirm that improved fire resistant properties are achieved for the normalized (N) condition. These results show the potential benefit of the proposed concept, i.e. controlling the solution / precipitation behaviour to allow strengthening precipitates to form during the heating associated with a fire. While this ‘active safety’ concept has been explored initially using a copper bearing steel, it is also considered to be potentially applicable to other strengthening precipitates such as micro-alloy carbo-nitrides in high strength low alloy (HSLA) steels.
The early studies reported above led to additional work to understand better the influence of microstructure and processing variables on the fire resistant properties. One of the study has focused especially on the niobium containing steel, to understand the influence of micro-alloying effects in the absence of the synergistic contributions of molybdenum. While commercial fire resistant steels contain molybdenum levels of the order of 0.5 %, it is desirable to minimize or eliminate the molybdenum addition if that is possible, due to its high cost at present.
Different studies have indicated that bainitic microstructures can show enhanced fire resistant properties, and follow up work has been conducted to compare ferritic, bainitic, and martensitic microstructures in the base steel, and ferrite + pearlite with bainitic microstructures in the niobium containing steel, also incorporating variations in the potential for NbC formation during heating.
Thermal treatments are carefully designed to separate the effects of niobium precipitation and general microstructure. The results indicated in a study that finer microstructures show higher strength at both ambient and high temperatures, and are an important contribution of niobium micro-alloying which is especially prominent in the constant load test response at high temperature. Increased NbC ‘precipitation potential’ during heating has shown an unclear effect for the bainitic microstructures, but is clearly beneficial for the ferrite + pearlite microstructures, and hence can offer some potential to develop the ‘active’ fire safety concept described above, using niobium micro-alloyed steels.
Different studies have shown that niobium is an effective alloying element for increasing high temperature strength, and this is particularly true in fire resistant steel applications where its contribution is frequently improved through molybdenum additions. Niobium forms carbo-nitride precipitates at higher temperatures than are normally associated with molybdenum carbide precipitation.
Weldability is an important consideration with respect to fire resistant steel developments, and the fire resistant steels developed to date (590 MPa class) are low carbon steels with typical additions of 0.02 % to 0.03 % niobium along with modest levels of molybdenum and manganese which reportedly provide outstanding heat affected zone toughness. These alloying elements hence provide attractive combinations of room temperature strength, resistance to softening at elevated temperatures up to around 600 deg C and good heat affected zone toughness.
Benefits of refined microstructure were considered potentially to be associated with the sub-structure present in steels transformed at lower temperatures. As a result, the effect on high temperature properties of thermo-mechanical processing to produce warm worked ferrite has been examined in later studies of the niobium containing alloy. Warm working is well known to increase the strength at room temperature, and the objective has been to evaluate whether stable sub-structures produced during thermo-mechanical processing at relatively high temperatures (i.e. above the temperatures frequently employed in fire resistant steel testing) can improve the fire resistant properties. Overall, the microstructure analysis confirms that low temperature finish rolling improves the development of ferrite substructure.